Convert two arrays to readonly

[ghost]

The cast is currently causing errors when running gulp local on the master branch. Turns out it's pretty easy to fix just this without any big API changes.

return emptyArray as string[]; // TODO: https://github.com/Microsoft/TypeScript/issues/16312

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src/server/server.ts(772,20): error TS2352: Type 'SortedReadonlyArray<never>' cannot be converted to type 'string[]'.